With a recent demand for a reduction in fuel consumption of automobiles, a conjugated diolefin-based rubber having low rolling resistance, and excellent wear resistance and failure characteristics, and also having steering stability represented by wet-skid resistance has been desired as a rubber material tire.
What is necessary to reduce the rolling resistance of a tire is oust to lower the hysteresis loss of vulcanized rubber. As indexes for the evaluation of vulcanized rubber, impact resilience at 50 to 80° C., tan δ at 50 to 80° C., Goodrich heat build-up and the like are used. A rubber material having either high impact resilience at 50 to 80° C., or low tan δ at 50 to 80° C. or low Goodrich heat build-up is preferred.
As a rubber material having low hysteresis loss, natural rubber, polyisoprene rubber, polybutadiene rubber or the like are known. However, they have the problem of low wet-skid resistance.
As a method for reducing the hysteresis loss without impairing the wet-skid resistance, there has been proposed a method of introducing a functional group into a polymer chain end of a styrene-butadiene copolymer which is polymerized by use of an organic lithium initiator in a hydrocarbon sol vent and has a variety of structure. There are known a styrene-butadiene copolymer obtained by modifying or coupling a polymer chain end with a tin compound (patent document 1) and a styrene-butadiene copolymer obtained by modifying a polymer chain end with an isocyanate compound or the like (patent document 2). These modified polymers exert the effects of reducing the hysteresis loss without impairing the wet-skid resistance and providing excellent wear resistance and failure characteristics, particularly in a composition in which carbon black is blended as a reinforcing agent.
On the other hand, there has recently been proposed a method of using as a rubber material for tires a rubber composition in which silica or a mixture of silica and carbon black is blended as a reinforcing agent. A tire tread in which silica or a mixture of silica and carbon black is blended has low rolling resistance and good steering stability represented by wet-skid resistance. At the same time, however, the tire tread has the problem that the tensile strength and wear resistance of a vulcanization are low. The above-mentioned modified styrene-butadiene copolymer becomes a rubber material for tires excellent in wear resistance and failure characteristics in the composition using carbon black as the reinforcing agent. However, in the composition using silica as the reinforcing agent, the improving effect thereof is small.
For the purpose of improving the tensile strength and wear resistance of the vulcanizate in which silica or a mixture of silica and carbon black is blended as the reinforcing agent, rubber compositions each containing a polymer into which a functional group having an affinity for silica is introduced have been proposed. Patent document 3 proposes a method of allowing a silicon tetrahalide, a trihalosilane or the like to react to produce a polymer. Further, patent document 4 discloses a method for producing a polymer modified with a silane halide compound. Furthermore, patent document 5 discloses an alkylsilyl group-introduced diene-based rubber, and patent document 6 discloses a silyl halide group-introduced diene-based rubber. In addition, patent document 7 discloses a diene-based rubber into which a tertiary amino group and an alkoxysilyl group are introduced.
The physical properties are improved to a certain degree by using each of these modified polymers in the composition in which silica or a mixture of silica and carbon blacks blended. However, improvements in the tensile strength and wear resistance of the vulcanizate are not sufficient yet. In particular, when a mixture of silica and carbon black is blended, a reduction in hysteresis loss is also insufficient with an increase in the ratio of carbon black. Further, in general, a silica-containing composition is inferior in processability to a carbon black-containing composition, which poses the problem of high processing cost. When the above-mentioned polymer into which a functional group having an affinity for silica is introduced is used, the processability thereof is unfavorably apt to further deteriorate.
Modified polymers which have hitherto been known are mainly classified into two types, polymers suitable for carbon black blending and polymers suitable for silica blending. When the kind of reinforcing agent is chanced in the case of producing a tire or the like, it has been necessary to select again a rubber to be used. Further, when a mixture of silica and carbon black is blended, even the use of either of the types of modified polymers has increased or decreased the effect thereof in correlation with the mixing ratio of silica to carbon black.
In both the carbon black blending and the silica blending, amino group-introduced polymers are conceivable as effective modified polymers. For the carbon black blending, there are proposed (1) polymers each having an amino group introduced into a polymer chain end by use of a lithium amide initiator (patent document 8, patent document 9, patent document 10, patent document 11 and patent document 12) and (2) polymers each obtained by modifying a polymer chain end of a styrene-butadiene copolymer which is polymerized by use of an organic lithium initiator and has a variety of structure, with a nitrogen-containing compound such as a urea compound (patent document 13), a dialkyl aminobenzophenone compound (patent document 14 and patent document 15) or a lactam compound (patent document 16), Further, as polymers for silica blending, amino group-introduced diene-based rubbers are proposed in patent document 17, patent document 18 and patent document 19.
In each blending of the carbon black blending and the silica blending, the polymers obtained by these methods have achieved the improvements of various characteristics to a certain degree. However, the above-mentioned literatures mainly describe in detail the methods for introducing amino groups into the polymers, but make reference to nothing more than general matters with respect to the relationship between the structure of the polymer itself and each performance.
Patent Document 1: JP-A-57-55912
Patent Document 2: JP-A-61-141741
Patent Document 3: JP-B-49-36957
Patent Document 4: JP-B-52-5071
Patent Document 5: JP-A-1-188501
Patent Document 6: JP-A-5-230286
Patent Document 7: JP-A-7-233217
Patent Document 8: JP-A-59-38209
Patent Document 9: JP-B-5-1298
Patent Document 10: JP-A-6-279515
Patent Document 11: JP-A-6-199923
Patent Document 12: JP-A-7-53616
Patent Document 13: JP-A-61-27338
Patent Document 14: JP-A-58-162604
Patent Document 15: JP-A-58-189203
Patent Document 16: JP-A-61-43402
Patent Document 17: JP-A-1-101344
Patent Document 18: JP-A-64-22940
Patent Document 19: JP-A-9-71687